Optical or photonic devices are becoming increasingly prevalent in many areas of technology today. For example, many communications systems include fiber optic cables and associated optical components for transmitting and receiving optical signals over the cables. Such optical systems provide very high data transfer rates and thus allow large amounts of data to be transferred very quickly, as will be appreciated by those skilled in the art.
In modern optical systems, optical components are integrated on a single substrate to thereby form compact, multifunctional, optical integrated circuits. These optical integrated circuits are analogous to electronic integrated circuits in which electronic components are formed and interconnected on a substrate to perform a desired function. In electronic integrated circuits, multiple layers are formed on the substrate and electronic components and required interconnections among such components are formed in these layers. The use of multiple layers allows more components to be formed on a single substrate, and also allows for easier and more efficient interconnection of such components.
Ideally, optical integrated circuits would also utilize multiple layers for the same reasons as electronic integrated circuits, namely to allow the formation of more optical components and easier and more efficient interconnection of the optical components. With optical integrated circuits, however, a unique problem is encountered that is different than electronic integrated circuits. When multiple layers are used, each layer must at selected points be coupled to one or more of the other layers to properly interconnect all the components formed in the layers. This is accomplished in a simple manner in electronic integrated circuits, as illustrated in FIG. 1 which shows a cross-sectional view of a portion of a conventional electronic integrated circuit 100 in which a via or electrical interconnect 102 electrically couples a first layer 104 to a second layer 106 formed on a substrate 108. In an electronic integrated circuit, the interconnect 102 is simply formed where required to interconnect the layers 104 and 106, which are conductive layers, and electrons flow between these layers through the interconnect.
In optical integrated circuits, waveguides are used in place of conductive layers and transfer optical energy or light between optical components. Unlike the electrons flowing in an electronic integrated circuit, light propagating through a layer cannot simply make a 90 degree turn and then propagate through an adjacent layer. For example, if the layers 102–106 in FIG. 1 correspond to waveguide layers in an optical integrated circuit, then light propagating through the layer 104 will not make a 90 degree turn and thereafter propagate through the interconnect 102 and into the layer 106. In fact, light propagating through the layer 104 would be confined to this layer and would not enter the interconnect 102 at all, as will be appreciated by those skilled in the art.
As a result of the problems associated with interconnecting multiple layers in optical integrated circuits, currently such circuits are limited to a single layer. This increases the cost and size of the circuits while limiting their functionality. The formation of “micro mirrors” to direct light from one layer to another has been proposed, but such an approach complicates the manufacture of the optical integrated circuit, which affects the cost and reliability of the circuit.
There is a need in optical integrated circuits to interconnect multiple layers so that light can propagate from one layer to another and thereby allow multilayer optical integrated circuits to be formed.